Pin tumbler lock cylinders, generally speaking, are susceptible to attacks by picking or bumping. In a picking attack, a first tool or torque wrench is inserted into the plug assembly keyway and a small threshold rotational torque is applied and held. A second tool or pick is inserted in the keyway and manipulated to successively move the key followers and associated cylinder pins so that the cylinder pins rise above the shear line between the cylinder body and plug assembly. The torque on the plug assembly will cause a slight misalignment of the respective key follower and cylinder pin bores, which will prevent the cylinder pins from falling back down across the shear line. During this process, the attacker must sense by feel the cylinder pin rising above the shear line and the amount the plug rotates, and apply greater or lesser torque to keep the “set” of the picked pins while feeling for the next pin's relationship to the shear line. In a bumping attack, the attacker inserts a special “bump” key into the keyway and applies a threshold rotational torque. Then the attacker applies at least one axial blow to the bump key. This shock causes the cylinder pins to jump above the shear line; as the pins rise, they separate as the shock is transferred from the bottom pin to the top pin, and the applied torque will turn the plug assembly before the cylinder pins can be driven back into place by their respective springs. However, too much applied torque will “crush” a cylinder pin at the shear line, which will then absorb the shock of the applied axial blow, and the cylinder pins won't jump. Another form of attack is to “impression” the lock mechanism.
The common denominator in all three types of attacks is applying, maintaining and modulating a rotational torque to the plug assembly and sensing it throughout the process.
One approach to defeating lock picking is by using spool pins, namely pins with an outer cylindrical shape and a smaller, inner cylindrical shape, to create false shear lines so that attackers think they've successfully moved a cylinder pin above the shear line. Unfortunately, the more spool pins are used to replace the cylindrical cylinder pins, the more likely that one or more spool pins will become unstable in the mechanism, and jam the lock, even when a valid key is used to unlock the mechanism.